The ultrasound imaging technique is a technique in which ultrasonic waves, which are inaudible sound waves and are commonly sound waves with frequencies of 20 kHz or more, are used to image the interior of a test object including human bodies in a noninvasive manner. For example, a medical ultrasound imaging apparatus transmits ultrasonic beams from an ultrasound probe to the interior of the body of a test object along transmission scanning lines, and receives echo signals from the interior of the body. A reception beam former generates signals, which are generated by phase-adding the received signals of a plurality of ultrasonic elements, at each of a plurality of received focal points on reception scanning lines. An image processing unit processes these phase outputs to generate an ultrasound image.
In the case where it is desired to form images by high speed imaging on the ultrasound imaging apparatus, limits are imposed on an increase in the operation speed in phase-addition by the reception beam former. Thus, a method is used, in which the number of times of transmission performed in a certain time period is decreased. In this case, in order to maintain the same imaging range (the viewing angle and the depth of the visual field), it is necessary to widen the interval between transmission scanning lines in the imaging range. In order not to extremely degrade the spatial resolution even though the interval between the transmission scanning lines is widened, a technique is known in which multidirectional reception scanning lines are set in parallel with each other to one transmission scanning line (unidirectional transmission multidirectional parallel simultaneous reception).
However, it is known that in the case where multidirectional parallel simultaneous reception is performed as described in Patent Literatures 1 to 4, stripes are produced on an ultrasound image, because the signal level of the phase output at the received focal point is varied between the adjacent reception scanning lines. In order to decrease these stripes, Patent Literature 1 discloses a method in which the phase output is subjected to weighted addition between reception scanning lines. Patent Literature 2 discloses a technique in which the gain of the signal of a reception scanning line is adjusted to remove stripes. Patent Literature 3 discloses a technique in which the position of a transmission beam is shifted for each of the frames of an ultrasound image, and the mean value is calculated between image frames to remove stripes. In a technique of Patent Literature 4, a notch filtering process in the azimuth direction is performed on an ultrasound image to remove stripes.
On the other hand, Patent Literature 5 discloses a method in which some ultrasonic elements are driven to perform the first time transmission and reception, and in the second time transmission, other ultrasonic elements are driven in the same direction to perform transmission and reception, and a reception focusing process (phase-addition) is performed together with the received signals obtained in the first time reception and the received signals obtained in the second time reception. Thus, even though the circuit scale of the ultrasound imaging apparatus is decreased, image quality equivalent to that of an apparatus with a large circuit scale can be secured.